The present invention relates to scanning devices for decoding symbols, and more particularly to a method for decoding symbols using a high resolution image sensor.
Encoded symbols such as ID bar codes, 2D bar codes and symbols, such as data matrixes, are commonly found in retail, industrial, and other applications for identifying labeled goods, products, or components. Bar codes are symbols that comprise a series of alternating white and black elongated bars or modules which are aligned to define a code. Data matrixes comprise a plurality of black and white cells which are arranged in a two dimensional code. Both of these types of codes, as well as various other symbols known in the art, can be found in applications for identifying goods, applied either to a label or printed directly on a part or component.
Devices for reading encoded symbols typically employ an illumination device for shining light on the symbol and a camera module for detecting the reflected light. The camera module typically has a fixed focal distance and a fixed aperture, providing a fixed field of view (FOV). The sensor in the camera module is arranged as an array of pixels defined by a row and column location in the sensor, and typically employs a low resolution sensor having a VGA resolution of about 640×480 pixels. In operation, the scanning device illuminates the symbol, and the camera module detects image data as reflected light from the illuminated area in the field of view. A decoding algorithm is employed to decode the symbol based on the acquired data.
The decoding algorithms used in these devices require a certain number of pixels per symbology element bar or cell for accurate decoding. When the FOV is fixed, as is typically found in current devices, there is therefore a direct relationship between the resolution of the sensor (in pixels per row/column) and the smallest readable code (measured in mm/module for bar codes and mm/cell for matrix codes). To provide the appropriate resolution, and both fast and accurate decode times for different types of symbols, readers are therefore typically specialized for a specific application and include lenses and/or focal distances which are fixed based on the expected application and the expected type of symbol to be read.
These specialized devices are useful for work stations where a single type of symbol is expected to be read under stable environmental conditions. However, it is often desirable to read different types of marks at a single station. To allow for reading of different types of symbols under varying environmental conditions, therefore, handheld readers are also available which use autofocus or bifocal lenses. These devices extend the reading range of the scanning device and therefore provide a variety of magnifications, thereby providing more versatile scanning capable at reading different types of symbols. Scanning devices including autofocus and bifocal lenses, however, can also be expensive and difficult to use. Autofocus and bifocal devices, for example, are highly dependent on the skill of the operator, as the operator must manually position the reader depending on the type of code being read. Furthermore, as the reader is moved further away, proper illumination of the symbol becomes problematic, rendering accurate reading difficult. These devices, therefore, require frequent re-positioning, are time-consuming to use, and can also be inaccurate.